1. Field of the Invention
The present invention relates to an image-sensing device having a photoelectric conversion means that outputs a signal logarithmically proportional to the amount of incident light.
2. Description of the Prior Art
A conventional area sensor having photosensitive devices such as photodiodes outputs a signal that is linearly proportional to the brightness of the light incident on the photosensitive devices. When a subject is shot with such a linear-conversion-based area sensor (hereinafter referred to as a “linear sensor”), no brightness data is obtained outside the roughly two-digit brightness range within which the linear sensor can effectively perform image sensing (this brightness range will hereinafter be referred to as the “shootable brightness range”). Here, this brightness range is represented as Lmax/Lmin, assuming that the brightness of the subject distributes from a minimum value Lmin [cd/m2] to a maximum value Lmax [cd/m2], and the range of the output of the linear sensor that corresponds to the range of the shootable brightness range is called the “dynamic range”.
Accordingly, when the signal from this linear sensor is reproduced as an image on a display or the like, the displayed image suffers from flat blackness in low-brightness portions thereof and saturation (flat whiteness) in high-brightness portions thereof outside the shootable brightness range. It is possible to alleviate such flat blackness or saturation by shifting the shootable brightness range. However, this requires varying the aperture value or shutter speed of a camera, or the integral time for which to allow light in, and thus spoils ease of use.
On the other hand, the applicant of the present invention once proposed an area sensor (hereinafter referred to as a “LOG sensor”) provided with a light-sensing means that outputs a photocurrent proportional to the amount of incident light, a MOS transistor to which the photocurrent is fed, and a bias means for biasing the MOS transistor in such a way that a subthreshold current flows therethrough, so that the photocurrent is converted logarithmically (refer to U.S. Pat. No. 4,973,833). This LOG sensor outputs a signal whose level is natural-logarithmically proportional to the brightness of incident light, and thus offers a wide shootable brightness range that corresponds to a five- to six-digit brightness range. This permits, even when the brightness distribution of a given subject tends to shift, the brightness distribution of the subject to lie most probably within the shootable brightness range.
However, a typical subject has a two- to three-digit brightness range, and therefore, if it is shot with a LOG sensor that offers a five- to six-digit dynamic range, the shootable brightness range is too wide relative to the actual brightness distribution of the subject, and this tends to spoil the sharpness of the obtained image.